Nowadays, photon-counting X-ray detectors are known as X-ray detectors used in X-ray CT apparatuses. Each of X-ray detection elements included in a photon-counting X-ray detector outputs a detection signal that enables counting of incident X-ray photons, and enables measurement of an energy value (keV) of each of the X-ray photons. When a photon-counting detector is used, calibration is indispensable to correct variation in X-ray energy sensitivity between the X-ray detection elements.